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234 UNIT C

What you will learn:• There are relationships among voltage, current, and resistance.

• Different meters measure electrical energy quantities in different ways.

• Electrical energy is produced, transferred, and converted into other formsof energy, which will help you handle electrical devices safely.

• Series circuits and parallel circuits operate differently and are used indifferent applications.

Each light bulb in this image of Saskatoon is lit because of the movement of electrons throughthe wires that connect the bulbs.

Learning Vocabulary in Context

This chapter contains many new terms related to electrical energy. Skimand scan Section 7.1 for the ways that vocabulary is supported. Wherecan you find definitions? How are unfamiliar terms highlighted in the text?What special features explain terms or words? Begin a personal list ofunfamiliar terms, adding definitions as you find them in the chapter.

Before Reading

Current electrical energy is the flowof electrons in a closed circuit.

By the end of this chapter,you will:

• demonstrate and analyzecharacteristics of staticelectric charge and currentelectricity, includinghistorical and culturalunderstanding

• analyze the relationshipsthat exist among voltage,current, and resistance inseries and parallel circuits

Key Terms• alternating current (AC)• ammeter • ampere (A)• battery • circuit • circuitbreaker • circuit diagram• current electrical energy• direct current (DC)• dry cell • electric current• electrochemical cell• electrodes • electrolyte• fuse • ground fault circuitinterrupter (GFCI) • load• multimeter • Ohm’s Law • ohmmeter • ohms (Ω)• parallel circuit • primary cell• resistance • resistor• secondary cell • series circuit• short circuit • superconductor• switch • transistor • volt (V)• voltage • voltmeter • wet cell

Outcomes

7

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07_sk_sci9_se_ch07:Layout 1 3/28/11 8:03 AM Page 234

Fuel CellsWhat do houses, buses, and laptops have in common? They canall be powered using fuel cells.

Fuel cells generate electrical energy from a chemical reactionwith a fuel, such as hydrogen (Figure 7.1). Using oil and otherfossil fuels, such as coal, for energy produces pollution. Most fuel cells create no pollution and actually produce pure water as a by-product.

A fuel cell is not used up like an ordinary cell (i.e., a battery)would be because as the electrical energy is produced, more fuel is added by taking it from the air. Currently, much of the energyproduced by fuel cells is wasted as heat, but their design continuesto be refined to make them more efficient.

In 2010, a fleet of 20 Canadian-made fuel cell-powered electricbuses began operating in Whistler, BC (Figure 7.2). The buseswere fully implemented for the 2010 Olympic and ParalympicWinter Games to be able to manage the huge increase in touristswithout a huge increase in pollution. These buses produce 60 percent less greenhouse gases, compared to diesel buses.

Here is a summary of what youwill learn in this section:

• Voltage is the difference inelectric charge between two points.

• Current is the rate of movementof electric charge through aconductor. Resistance is theability of a material to resist the flow of electric charges.

• An electrochemical cellgenerates voltage by creatingan imbalance of charges between its terminals.

• Models can be mental,mathematical, or acombination. Scientific models can help youcommunicate your ideas.

Voltage, Current, and Resistance

Figure 7.1 A fuel cell converts chemical energy into electrical energy. The fuel cell shown hereis only slightly smaller than this textbook.

7.1

2357.1 Voltage, Current, and Resistance©P

Figure 7.2 New fuel cell-poweredbuses in Whistler, BC


Zero Emission Bikes

Launched in London, UK, in March 2011, fuel cellscooters are taking Europe by storm. These zero emissionbikes are taking advantage of Europe’s existing hydrogenfuelling stations to provideinexpensive transportation for the European Community.

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236 UNIT C Characteristics of Electricity©P

Buses are an excellent choice for fuel cell technology. They donot need an infrastructure of hydrogen fuelling stations as carswould, since they all return to one central depot for maintenanceand refuelling. There are many bus systems in such cities asLondon, Barcelona, and Hamburg in Europe, and Perth inAustralia that are already converted to fuel cell technology.

Fuel cells combine oxygen in the air with hydrogen in the cells to power electric vehicles. One day, fuel cells may beused to power smaller devices such as laptop computers. Fuel cells could also be integrated with current power generatingstations like wind, solar, and hydro. These stations often produceelectrical energy when it is not needed. That electrical energy can be stored in fuel cells for future use.

C10 Quick Science

In this activity, you will use a combination of wires,light bulbs, and a hand-operated generator to makea bulb light up.

PurposeTo discover how to make flashlight bulbs light upusing a hand-operated generator

Light the Lights

3. If time allows, try other arrangements for step 1and step 2.

Questions4. Describe how to use wire and a hand-operated

generator to make one bulb light up. Include alabelled sketch of an arrangement that worked to light up the bulb and one that did not light up the bulb.

5. Describe how to use wire and a hand-operatedgenerator to make two bulbs light up. Include alabelled sketch of an arrangement that worked to light up the bulbs and one that did not lightup the bulbs.

Pose New Questions6. Where do you think the energy comes from to

light the bulb(s)?

7. Using your observations from this activity, whattestable question could you write that yourresults from this activity would answer?

8. What other questions do you have about yourobservations? Write a testable question for anactivity that would answer your questions.

CAUTION: Disconnect the wires if they get hot.

• five insulated copper wires with both ends bare

• hand-operated generator

• two 2.0-V flashlight bulbs

Materials & Equipment

Procedure1. Use wire and the hand-operated generator to make

one bulb light up. Record your arrangementusing a labelled diagram or sketch.

2. Use wire and the hand-operated generator to maketwo bulbs light up. Record your arrangementusing a labelled diagram or sketch.


Current Electrical EnergyAs you saw in Chapter 6, the small static electrical energydischarges you have felt from a sweater is similar to the hugestatic electric discharges of lightning. Unfortunately, static electriccharges are not useful for operating electrical devices. To operateelectrical devices, you need a steady flow of electric charge.

Unlike static electric charges and discharges, a flow of electriccharge continues to move as long as two conditions are met. First,the flow of electric charge requires an energy source. Second, theelectric charge will not flow unless it has a complete path to flowthrough. This path is called an electrical circuit. The flow ofelectric charge in a circuit is called current electrical energy.

Electric CircuitsAn electrical circuit includes an energy source, a conductor, and aload. An electrical load is a device that converts electrical energy toanother form of energy. For example, in Figure 7.3, the light bulbis the load. It converts electrical energy to light and heat energy.

Many electric circuits also include a switch. A switch is a devicethat turns the circuit on or off by closing or opening the circuit.When the switch is closed (Figure 7.3), the circuit is complete andelectric charge can flow. An open switch (Figure 7.4) means thereis a break in the path, so the electric charge cannot flow throughthe circuit. The circuit is turned off when the switch is open.

The light switch is a common example of a switch we use everyday. It controls the power to the light. Other examples of commonswitches might also control electric ceiling fans or garage doors.


Suggested Activity •C11 Problem-Solving Activity on page 245

electrical load

switch

energy source

conductingwires

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Figure 7.3 A closed electric circuit

Figure 7.4 An open electric circuit

electrical load

switch

energy source

conductingwires

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electrical lloadoadad

witch

energy sou


The Origin of Volt and Voltage

The electrochemical cell wasfirst presented to the RoyalSociety of London in 1800 bythe Italian physicist AlessandroVolta. The words “voltage” and“volt” are named in his honour.

infoBIT VoltageEach electron in a conductor, such as a copper wire, has electricalenergy stored within it. When the conductor is connected to anenergy source, the electrons flow through the conductor. Theenergy source provides more energy to each electron, whichenables them to move through the conductor.

The difference in electrical energy between two points in acircuit is called the voltage. The higher the voltage in a circuit, thegreater the stored electrical energy that is provided to each electron.

Measuring VoltageThe voltage in a circuit is always measured between two locationsin that circuit. A voltmeter is used to measure voltage (Figure 7.5).The voltmeter is showing the difference between the energy levelsat the two points (voltage is also sometimes referred to as potentialdifference). The SI unit for measuring voltage is the volt (V).

How Electrons Transfer Energy in a CircuitWhen you turn on the light switch on a wall, you close the circuitand the light comes on immediately. How does the electric chargeget from the switch to the light bulb so fast? It may surprise youto learn that individual electrons do not travel from the switch tothe bulb when the switch is turned on. Picture electrons in a wireas being like water in the water pipes in your house. The waterpipes are normally full of water so when you turn the tap on,water comes out of the tap immediately (Figure 7.6).


Figure 7.5 A voltmeter

faucet lever down in “off” position faucet lever up in “on” position

Figure 7.6 Electrons in a wire are like the water in the water pipes in your home. Since the pipes are already full,water comes out of the tap as soon as you turn it on.


Electrons in a wire work in a similar way. When an energysource is connected to a circuit, electrons in the conductorinstantly “push” or repel other electrons nearby because of their electric charges.

When one electron moves at one end of the wire, it pushes the next one, which pushes the next one, and so on. This processhappens instantly. By pushing the first electron, you make the last electron move. That is why when you flip the switch, thelight goes on instantly even though the electrons themselves have not moved from the switch to the light bulb.

CurrentElectric current is a measure of the amount of electric chargethat passes by a point in an electrical circuit in a given timeinterval. Think of the steady flow of electric current as being like water flowing in a stream. The water keeps on flowing unlessits source dries up. As long as the energy source continues, theelectrons continue to flow. Because the current flows in only one direction, it is called direct current (DC).

The current that flows through cords plugged into the wallsockets in your home is called alternating current. Alternatingcurrent (AC) flows back and forth at regular intervals calledcycles. This is the current that comes from generators and iscarried by the power lines to your home.

Measuring CurrentCurrent in a circuit is measured using an ammeter (Figure 7.7).The unit of electric current is the ampere (A). An ampere is ameasure of the amount of charge moving past a point in thecircuit every second.


Figure 7.7 These ammeters show a reading of 0.50 A. The meter on the right has amperes onthe scale below the black curved line.

The Origin of Ampere

“Ampere” and “ammeter” arenamed in honour of André-Marie Ampère (1775–1836), a French physicist who studiedelectricity and magnetism.

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Current Electrical Energy and Static Electric ChargeCurrent electrical energy differs from static electric charge becausecurrent electrical energy is the flow of electrons in a circuit througha conductor. Static electric charge is the electric charge that buildsup on the surface of an object. Static electric charge dischargeswhen it is given a path, but does not continue to flow.

ResistanceResistance is the degree to which a substance opposes the flow of electric current through it. All substances resist electron flowto some extent. As you saw in Chapter 6, conductors, such asmetals, allow electrons to flow freely through them and have low resistance values. Low resistance is useful, for example, inpower stations that want the maximum of electric current to beconducted to homes and businesses. Insulators, such as plasticand wood, resist electron flow to a greater degree and have highresistance values. In the case of incandescent light bulbs, highresistance is important because the resistance is what causes the light bulb filament to emit light. Resistance is measured inohms (Ω) using an ohmmeter. An ohmmeter is usually part of a multifunctional meter called a multimeter (Figure 7.8).

When a substance resists the flow of electrons, it converts the electrical energy into other forms of energy, such as heat orlight energy. There is still the same number of electrons passingthrough the circuit, but they each have less energy. The moreresistance a substance has, the more energy it gains from theelectrons that pass through it. The energy gained by the substancemay be radiated to its surroundings as heat, light, sound, orkinetic energy (Figure 7.9).


Figure 7.8 Multimeters can be usedto measure voltage, current, orresistance.

The Symbol for Ohm

The symbol for ohm, Ω, is the Greek letter omega.

Kinetic Means Movement

“Kinetic” means active ormoving. Kinetic energy is theenergy of motion. Kinetic energydepends on how fast somethingis moving and its mass.

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Figure 7.9 When electrons pass through a resistor, such as the element on this electric heater,their electrical energy may be converted to heat, light, sound, or kinetic energy.


Conductivity VersusResistivity

The inverse of conductivity is resistivity; the inverse ofresistance is conductance.

infoBITResistance in a CircuitThe more resistance a component has, the smaller itsconductance. As you learned in Chapter 6, conductivity is theability of materials to allow electrons to move freely in them. For example, current in a circuit might pass through the filamentin an incandescent light bulb (Figure 7.10). The filament is aresistor, which is any material that can slow current flow. The filament’s high resistance to the electron’s electrical energy causes it to heat up and produce light.


SuperconductorsSuperconductivity can be seen in some metals and ceramic materials.Even more than regular conductors, superconductorshave no electrical resistance below a characteristictemperature. This means that electrons can travelthrough them freely when they are cooled totemperatures near absolute zero, �270°C. Theycan carry large amounts of electrical current forlong periods of time without losing energy asheat (thermal) energy. In addition to theseextremely low-temperature superconductors,scientists have now discovered that some substancesact as superconductors at temperatures above�243°C. Superconductivity may be used in thefuture for electrical power transmission. One ofits current uses is to make a superconductingmagnet in magnetic resonance imaging (MRI)machines in hospitals (Figure 7.11).

Suggested Activities •C12 Inquiry Activity on page 246C13 Inquire on Your Own on page 247

filament

Figure 7.10 The filament in an incandescent light bulb is an example of a resistor.

Figure 7.11 This magnetic resonance imaging (MRI) machineuses a superconducting magnet.


Resistors and VoltageResistors can be used to cause the electrons to transfertheir energy to a different type of energy. When youwork with resistors, you should always be aware thatthey can heat up and cause burns. Use caution whenhandling them.

In a circuit, electrons have a higher voltage as theyenter a resistor compared to when they leave the resistorbecause they use up some energy in passing through theresistor. You can picture electrons entering a resistor asbeing at the high end of a ramp, where they have a lot of (gravitational) potential energy. In this analogy,electrons leaving the resistor are at the bottom end of the ramp, where their potential energy has beenconverted to another form of energy (Figure 7.12).

Types of ResistorsA wide variety of resistors are made for differentapplications, especially in electronics (Figure 7.13). Forexample, televisions contain dozens of different resistors.

Resistors are needed in equipment where one powersupply is used in different functions, like in yourcomputer. There are many different circuits withdifferent power demands.

A resistor is used if a part of a circuit needs lesscurrent than what the power supply is providing. Aresistor may also be needed if the current needs to vary.For example, dimmer switches would use a variableresistor. If you lower the light, you are increasing theresistance. Another example would be the volumecontrol for televisions.


Learning Checkpoint

1. How is current electrical energy different from the build-up of static electric charges?

2. When you walk into a dark room and turn the light on, do the electronstravel all the way from the switch to the light? Explain your answer.

3. Voltage has been described as similar to the water in the water pipes inyour home. What other analogy can you think of that is similar to voltage?

4. What is electric current?

5. What does “resistance” refer to in terms of electron flow?

Figure 7.13 Resistors come in many shapes andsizes. The type of material the resistor is made from is one factor that affects its resistance.

potential energy convertedto another form of energy

high potential energy

Figure 7.12 An electron entering a resistor is similarto a ball at the high end of a ramp, where potentialenergy is greater.


Definitions in Context

Often, unfamiliar terms aredefined right in the text thatyou are reading. You do notneed to look them up in aglossary or dictionary. Lookfor the boldfaced words, andthen find the definition in thesentence either before orafter the term. Add wordsand definitions to yourpersonal list of terms.

During ReadingElectrochemical CellsA simple and convenient energy source is a battery. A batteryis a combination of electrochemical cells. Each electrochemicalcell is a package of chemicals that converts chemical energy intoelectrical energy that is stored in charged particles. A simpleelectrochemical cell includes an electrolyte and two electrodes:

• An electrolyte is a liquid or paste that conducts electriccharge because it contains chemicals that form ions. An ionis an atom or a group of atoms that has become electricallycharged by losing or gaining electrons. Citric acid is anexample of an electrolyte.

• Electrodes are metal strips that react with the electrolyte.Two different electrodes, such as zinc and copper, are used in a battery.

As a result of the reaction between the electrolyte and electrodes,electrons collect on one of the electrodes, making it negativelycharged. The other electrode has lost electrons, so it becomespositively charged (Figure 7.14).


Wet Cells and Dry CellsAn electrochemical cell that has a liquid electrolyte is called a wet cell. Wet cells are often used as an energy source for cars and other motorized vehicles because they are less costly than dry cells and most can be easily recharged. An electrochemicalcell that uses a paste instead of a liquid electrolyte is called a dry cell or primary cell (Figure 7.15). You use dry cells inflashlights, hand-held video game devices, cameras, and watches.A dry cell is a more practical choice in these devices since it canbe operated in many different positions including upside downwith no concern about the electrolyte spilling.

Figure 7.14 The citricacid in the grapefruit is the electrolyte.Electrons collect on thezinc electrode, leavingpositive charges onthe copper electrode.The meter measuresthe flow of electrons.

copper electrode (�)

A

B

E

F

C

D

A –

B –

C –

D –

E –

F –

zinc powder and electrolyte,where electrons are released

electron collecting rod

separating fabric

manganese dioxide and carbon,where electrons are absorbed

negative terminal, where electrons leave

positive terminal, where electrons return

Figure 7.15 An alkaline dry cell

zinc electrode (�)


Recycling and Re-energizing Dry CellsEventually, the chemicals in a dry cell or primary cell are used upand can no longer separate charges. When you are finished usinga dry cell, you should recycle it rather than discard it (Figure 7.16).Dry cells can contain toxic materials and heavy metals such asnickel, cadmium, and lead, which are harmful to the environmentand living organisms. Household dry cells and batteries areresponsible for over 50 percent of all the heavy metals found inlandfills. Some dry cells can be re-energized, and are referred to as “rechargeable” or secondary cells. Chemical reactions in arechargeable cell can be reversed by using an external energysource to run electrical energy back through the cell. The reversed flow of electrons restores the reactants that are used up when the cell produces electrical energy. Secondary dry cells can bereused many times and have less impact on the environmentthan non-rechargeable dry cells. Primary cells are superior interms of shelf life, as they hold their charge for a longer period of time when not in use. However, secondary cells have greaterlong-term benefits since they can be used over and over again.


Many rechargeable dry cellsare available, such as NiCd,NiMH, and lithium ion. Using library or Internetresources, research thedifferent types and comparetheir composition, lifetime,cost, and ability to hold charges.Display your results in a chart or graphic organizer.

reSearch

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Figure 7.16 An electrochemical cell gives electrons electrical energy, or voltage.

1. What are some examples of electrical loadsthat you can see in your classroom?

2. Describe the two main components of anelectrochemical cell.

3. Explain how electrons flow in a circuit.

4. (a) What device measures voltage?(b) What are the units for measuring voltage?

5. (a) What device measures current?(b) What are the units for measuring current?

6. (a) What is the function of an electricalload in a circuit?

(b) List four examples of electrical loadsand the type of energy transformationin each.

7. What does resistance refer to in a circuit?

8. What is the role of a resistor in a circuit?

CHECK and REFLECT7.1

9. Why must a circuit be closed in order for a current to flow?

10. Make a list of similarities between the flowof water and an electric circuit.

11. (a) Using a graphic organizer of your choice,compare the benefits and drawbacks of both primary and secondary cells.

(b) Referring back to your graphic organizerfrom (a), which cell would you recommendfor a digital camera? Explain.

12. What is the difference between anelectrolyte and an electrode?

13. Why should dry cells be recycled ratherthan thrown in the garbage?

14. What do you now understand aboutcurrent electrical energy that you did not know before reading this chapter?


245Chapter 7 Current electrical energy is the flow of electrons in a closed circuit.©P

C11 Problem-Solving Activity

Initiating and PlanningCan an electric motor be built from everyday materials?

Making a Simple Electric Motor

SKILLS YOU WILL USE■ Recording results■ Identifying potential impacts

Toolkits 3, 11

5. Use the wood block for the base. Use the screwsto mount the bent safety pins on the block so that the loops are facing each other and areabout 2 cm apart.

6. Use double-sided tape to secure the magnet tothe block between the two safety pins (Figure 7.18).Secure the battery holder to the block underneaththe magnet. Connect the wires from the batteryholder to the safety pins. Insert the battery intothe holder. You may need to give the armature a push to get the motor started.

Performing and Recording1. First, wind a coil of wire to create the part of

the motor that moves (armature). Wind it on a cylindrical form, such as a small battery to keep the shape even.

2. To make the coil hold its shape permanently, twist the free ends and wrap them around thecoil a couple of times or use electrical tape tosecure the free ends.

3. Hold the wire coil so that it is vertical. Carefully usethe tips of the scissors to scrape off the insulationfrom the top of the wire ends. You may wish touse the wood block to support the free ends.

4. Use pliers to bend two safety pins at the middleto make supports. The safety pins can conductelectrical energy to the armature while the loopsof wire on the safety pins can hold it up.

Figure 7.18 Step 6

Analyzing and Interpreting7. Adjust the angle of the motor so it can work in

either a vertical or horizontal position. Draw adiagram of your motor in the different positions.

8. Adjust the shape of the coils and see how theywork. Is the coil the best shape? Try squares,ovals, and so on. Make a chart showing each ofthe shapes you tried and write a short summaryof the results underneath them.

9. Vary the number of turns of wire in the coil. Doesan odd or even number of turns matter? Does thenumber of turns determine the speed? Make achart showing the number of coils and the results.

Communicate10. Can you make the motor do any work?

Demonstrate your results to the class.

• magnet wire

• electrical tape

• scissors

• pliers

• two large safety pins

• wood block

• screws

• screwdriver

• double-sided tape

• magnet

• battery holder with wires

• battery


Figure 7.17 Be careful when scraping off theinsulation. Only remove the insulation from the top half of the wire.



C12 Inquiry Activity

A physical model can help you understand a processor object that may be hidden or too large or small toview directly. Models can be mental, mathematical, or a combination. You can use a scientific model tohelp you communicate your ideas.

Initiating and PlanningHow can you use a model to help you understand theinteractions among voltage, current, and resistance?

Modelling Voltage, Current, and Resistance

SKILLS YOU WILL USE■ Observing and measuring■ Analyzing patterns

Toolkit 2

7. Record the time it takes to fill the beaker usingthe slightly pinched length of tubing.

8. Record the time it takes to fill the beaker using an open length of tubing.

9. Record the time it takes to fill the beaker using anopen length of tubing and the water turned on full.

Analyzing and Interpreting10. (a) How did the exit times compare for the tubes

in step 3 and step 5?

(b) How would you explain any difference in times?

11. What part of this activity modelled electric currentin a circuit?

12. (a) How does the size of the opening in the tubingaffect water flow?

(b) Relate the size of the opening of the tubing to resistance in wires.

13. (a) How does how far a tap is opened affectwater flow through the tubing?

(b) Relate how far a tap is opened to voltage in a circuit.

Communication and Teamwork14. How did you divide up the tasks in this activity to

ensure that all members of your group wereincluded and contributed equally? Would youorganize things differently if you were to performthe activity again?

15. How accurately do you think this activitymodelled interactions among voltage, current,and resistance? What modifications would makethe activity more accurate?

Performing and Recording1. Create a data table like the one below. Give your

data table a title, and use it to record your results.

2. Attach one end of the tubing to the tap. Put theother end into the sink as far from the tap as the tubing will reach without bending.

3. Turn on the water to a medium flow. Record the time it takes for water to exit the tubing.

4. Pinch the end of the tubing, and turn off thewater. Keep the end pinched.

5. Simultaneously, turn on the water to a mediumflow and release the end of the tubing. Recordthe time it takes for water to exit the tubing into the sink.

6. While the water is running, pinch the end of thetubing slightly. Observe what happens to the flow.

Time to Exit Empty Tube (s)

Time to ExitPinched Tube (s)

Time to Fill Beakerwith Pinched Tube (s)

Time to Fill Beakerwith Open Tube (s)

Time to Fill Beakerwith Water on Full (s)

• 50-cm or longer lengthof rubber tubing

• water tap and sink

• stopwatch

• 1000-mL beaker




C13 Inquire on Your Own

QuestionHow do the conductivity of different solutions compare?

Investigating Conductivity

SKILLS YOU WILL USE■ Using appropriate equipment

and tools■ Designing an experimental

procedure

Toolkits 1, 11

3. Put 50 mL of distilled water into a 250-mL beaker.

4. Place the metal tips of the conductivity tester in the distilled water (Figure 7.19). Record theconductivity reading of the distilled water in yourtable. If your conductivity tester is a light bulb,describe the brightness of the bulb.

5. Repeat steps 3 and 4 with 50-mL samples of tap water, salt water, vinegar, copper(II) sulphatesolution, and any other solutions your teacherprovides for you to use. After each conductivitymeasurement, empty the beaker as directed byyour teacher and rinse it with distilled water. Also,wipe off the tips of the conductivity tester. Makesure that you insert the tips to the same depth in each solution.

6. Clean up your work area. Make sure to followyour teacher’s directions for safe disposal ofmaterials. Wash your hands thoroughly.

Part 27. Plan an investigation to compare the conductivity

of other solutions. Have your teacher approveyour plan, and then conduct your investigation.

8. How did you determine whether there weredifferences in conductivity between the solutionsyou tested?

9. Rank the substances in order of high conductivityto low conductivity.

10. How did your results compare with your predictions?

11. Make a hypothesis about why there were differencesin conductivity between the solutions.

12. Write a summary of your results that answers thequestion “How does the conductivity of differentsolutions compare?” Present your findings in theformat of your choice.

Design and Conduct Your Investigation

Part 11. Design a data table to record your predictions

and your conductivity readings of the solutionsyou will test. Give your table a title.

2. Predict which solutions will be the best conductorsand which will be the poorest conductors.Record your predictions and the characteristicson which you are basing your predictions.

Figure 7.19 Conductivity tester

• distilled water

• 250-mL beaker

• conductivity tester

• tap water

• salt water

• vinegar

• copper(II) sulphate solution

• other solutionsprovided by yourteacher

• paper towels



Designing CircuitsComputers and devices such as toy robots (Figure 7.20) havecomplex circuits. Other electrical devices, such as flashlights or hair dryers, have much simpler circuits, often designed in a loop.If you take a flashlight apart, you will probably find a light bulb,some wire, some batteries, and a plastic casing to hold and protectthe electrical parts. Flashlights are easy to build with readilyavailable materials and can be assembled efficiently.

However, a simple loop is not always the best design when thereare many different components in the circuit. Designers make surethat one component does not depend on another. For example, itwould be very frustrating if the toy robot or your computer stoppedworking completely just because one of its LED indicators went out.These devices have many electrical paths so that if one componentstops working, the rest of the device will continue to function.

Tiny CircuitsConventional switches and other electrical components arepractical and convenient for simple electrical devices. However,for the tiny circuits in advanced electronic devices such ascomputers, transistors must be used instead.

248 UNIT C Characteristics of Electricity


• A circuit diagram is a model of an electric circuit.

• An ammeter is hooked up inseries to measure current. A voltmeter is hooked up inparallel to measure voltage.

• In a series circuit, the currentis constant and the voltagesacross resistors add up to thetotal voltage supplied by theenergy source.

• In a parallel circuit, the voltagesacross loads are constant andthe currents on each path addup to the total current leavingthe energy source.

Series Circuits and Parallel Circuits

Figure 7.20 These toy robot dogs are controlled by electric circuits.

7.2

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A transistor is a tiny device that acts as a switch oramplifier in a circuit. Transistors are often referred to assolid-state components because they are made of solidmaterial with no moving parts. Most transistors areconstructed with three layers of specially treated silicon.These layers are arranged so that a small amount of voltagethrough the middle layer controls a current between theouter layers. In this way, transistors can act as switches.

Microcircuits (also called integrated circuits) aremade up of microscopic transistors and other electricaldevices. A microcircuit is simply a circuit on anextremely small scale. Microcircuits, or microchips,regularly contain more than a million components per square centimetre (Figure 7.21).

2497.2 Series Circuits and Parallel Circuits©P

Figure 7.21 A microcircuit is usually called a “chip” or a “microchip.”

C14 Quick Science

Current exists when a circuit is complete. If there is a break in a circuit, due to a burned-out bulb, forexample, the current cannot continue. In this activity,you will investigate how to keep electric chargeflowing through a circuit even though one bulb may be burned out or missing.

PurposeTo compare the flow of electric charge in differenttypes of circuits

Keep the Lights On

2. Circuit B: Connect all three bulbs so that youcan remove one bulb without disconnecting thewires and still have the other bulbs stay on.Make a labelled drawing of your set-up.

Questions3. (a) What would happen to the other two bulbs

if you removed one bulb in Circuit A?

(b) Why would this happen?

4. Why did the other two bulbs stay lit when youremoved one bulb in Circuit B?

5. Draw a circuit that would allow you to removetwo bulbs and yet have the third bulb stay lit.Have your teacher approve your drawing. If timeallows, test your ideas by building the circuit.

Pose New Questions6. Compare your results with your classmates.

Write a testable question that you would like to answer based on the results.

• D dry cell

• five insulated copper wires with both ends bare

• three 2.0-V flashlight bulbs


CAUTION: Open the circuit if the wires get hot.

Procedure1. Circuit A: Using any of the materials, determine

how to connect three bulbs in a row so they alllight up. Make a labelled diagram of your set-up.


Symbol Component Function

wire conductor; allows electriccharge to flow

cell, battery electrical energy source;longer side represents thepositive terminal, shorter side represents the negative terminal

lamp (light bulb) specific load; convertselectrical energy to light and heat energy

resistor general load; convertselectrical energy to heat energy

switch opens and closes the circuit

ammeter measures current through adevice, connected in serieswith the device

voltmeter measures voltage across adevice, connected in parallelwith the device

Table 7.1 Some Common Circuit Symbols

CircuitsA circuit is a pathway that electric current follows. It must be in a complete loop in order for it to work properly. Circuits can bevery complicated, but contain the same basic parts. There is awire to allow the energy to flow, a load to convert the energy, forexample, to heat or light, and an energy source (Figure 7.22(a)and (b)). It may also have a switch to open or close the circuit.

Circuit DiagramsA circuit diagram is a model used by engineers and designers inorder to design and analyze an electrical circuit (Figure 7.22(c)).They use special symbols that show the components and electricalconnections in a circuit. A circuit diagram is not necessarily drawnto scale. Circuits can be tiny, such as in microcircuits, or as largeas your home, such as the circuit that connects a light switch, an overhead lamp, and your home’s electrical panel.

You can use the symbols in Table 7.1 to draw and interpretcircuit diagrams (Figure 7.23). Knowing the basic circuit symbolscan help you analyze existing circuits and make it easier tounderstand where the current flows and how a device functions.


Drawing Circuit Diagrams

Always use a ruler to drawstraight lines for the conductingwires and make right-anglecorners so that your finisheddiagram is a rectangle.

infoBIT

switch

load

electrical source

conducting wire

Figure 7.23 The four basic parts of a circuit


Series Circuits A series circuit is an electric circuit in which the componentsare arranged one after another in series (Figure 7.24). A seriescircuit has only one path along which electrons can flow. If thatpathway is interrupted, the whole circuit cannot function.

The amount of current is the same in all parts of a seriescircuit. However, if you add more resistors, you increase the totalresistance of the circuit. This decreases the current if the voltageremains the same. Adding an extra bulb to a series string of lightsmakes all the bulbs dimmer.

Electrons use up all their energy going around a series circuitno matter how many loads are in the circuit. Each load will usepart of the total voltage, depending on how much it resists theflow of electrons.

Parallel CircuitsA simple parallel circuit is an electric circuit in which devicesare arranged in parallel paths (Figure 7.25), although each electriccharge only follows one path.The points where a circuit dividesinto different paths or where paths combine are called junctionpoints. An interruption or break in one pathway does not affectthe other pathways in the circuit. Similarly, adding a new pathwaywith more resistors does not affect the resistance in any of theother pathways. In fact, adding extra resistors in parallel decreasesthe total resistance of the circuit. This might seem strange, butthink about how much less resistance there is when you drinkthrough two straws instead of one.

Most electrons will follow the path with the smallest resistance.Therefore, the amount of current is greater on the paths with thesmaller resistances (Figure 7.26).

Each electric charge has the same amount of energy, andelectrons must expend all their energy on the path they are on.This is why the voltage across parallel resistors will always be thesame, even though the resistors themselves are of different values.

Table 7.2 on the next page summarizes the characteristics ofcurrent and voltage in series and parallel circuits.


Figure 7.24 A series circuit has onlyone path along which current can flow.

junction point

Figure 7.25 In a simple parallel circuit,each component has a parallel pathfor current.

6.0 A

2.0 A

1.0 A

3.0 A

Figure 7.26 Loads of differentresistance that are connected inparallel have different currents.


Two Types of CircuitsWhat happens when one light bulb burns out in a long string ofdecorative lights? If the set of lights is wired in series, the currentmust flow through one light before it gets to another light. Whenone light burns out, all lights go out. The current cannot flow past a burned-out bulb because the pathway is interrupted.

If the set of lights is wired in parallel, the current takes severaldifferent paths. If a light on one path goes out, current does not flowon that path. However, there are other paths where the current doesflow and lights on those paths remain lit. Series circuits and parallelcircuits make up the circuits in your home and school. Some circuitsare combinations of series circuits and parallel circuits (Figure 7.27).These combinations help prevent problems such as the refrigeratorturning off because a light bulb burned out in a bedroom. It is animportant safety feature in a combination circuit to have someswitches wired in series, because it is sometimes necessary toturn off the electrical energy in part or all of a home (Figure 7.28).


Suggested Activities •C15 Problem-Solving Activity on page 254C16 Skill Builder on page 255C17 Inquiry Activity on page 256C18 Inquiry Activity on page 257

Figure 7.27 A combination circuit.The switch in this circuit can turn all the bulbs on or off.

Figure 7.28 A typical home has many parallel circuits.

Circuit Voltage Current Resistance

Series circuit Each load uses a portion ofthe total energy supplied bythe battery.

The current is the samethroughout a series circuit.

The current decreases whenmore resistors are added ifthe energy remains the same.

Parallel circuit Each load uses all theenergy supplied by thebattery.

The current divides intodifferent paths. A pathwaywith less resistance will have a greater current.

Adding resistors in paralleldecreases the total resistanceof the circuit if the energyremains the same.

Table 7.2 Voltage, Current, and Resistance in Series and Parallel Circuits

Taking Notes

A simple strategy for takingnotes is to create a two-columntable. List important ideas,concepts, and terms in theleft-hand column. Then, write the key information ordefinitions related to thoseideas, concepts, and terms in the right-hand column. A two-column table is a good study tool.

During Reading



1. In your own words, define the term circuit.

2. Draw the circuit symbol for

(a) a light bulb

(b) an ammeter

(c) a voltmeter

3. Draw a circuit diagram for a circuit thatincludes a resistor, a switch, conductingwires, and a battery.

4. (a) Draw a circuit diagram of the circuitshown here.


7. Calculate the voltage across the source ineach of these circuits.

Question 4

(a)(d)

(b)(c)– +

Question 7

12 V 12 V 12 V

4.0 V 6.0 V2.0 V

(b) Is this a series circuit or a parallelcircuit?

(c) How do you know?

5. Suppose two pathways in a simple parallelcircuit have different resistances. Will thecurrent in each pathway be the same?Explain.

6. What images or memory aids help youremember the differences between seriesand parallel circuits?

8. You have three light bulbs, each with adifferent resistor. The amount of currentthrough a bulb will affect how much light it emits.

(a) Will the order in which you hook up thelight bulbs in series affect the intensityof light that each emits? Explain.

(b) What happens when you hook up thebulbs in parallel?

9. Draw a circuit diagram that shows

(a) three resistors in series

(b) three resistors in parallel

(c) one resistor in series and two resistorsin parallel

(a)

(b)



C15 Problem-Solving Activity

Suppose that all the lights in your home wereconnected in one simple circuit. When you closed aswitch, every light would come on. When you openedthe switch, every light would turn off. This arrangementwould not be very practical for most uses. Instead,lights can be connected in a circuit in such a way that some can be turned on while others are turnedoff (Figure 7.29). In this activity, you will investigatehow to create such a circuit.

Initiating and PlanningHow can a circuit have lights turned on and offindividually?

Off and On

SKILLS FOCUS■ Designing, building, and testing■ Explaining solutions

Toolkits 3, 6

Performing and Recording1. Circuit A: Design and draw a circuit diagram

where the three bulbs can be either all on or all off.

2. Circuit B: Design and draw a circuit diagramwhere each of the three bulbs in the circuit can be turned off and on individually.

3. Circuit C: Design and draw a circuit diagram wheretwo bulbs can be turned off while one stays on.

4. Have your teacher approve your three circuitdiagrams. Then, hook up the circuits and test whether they work.

5. Clean up your work area.

Analyzing and Evaluating6. For each circuit, describe whether the lights were

hooked up in series, in parallel, or in a combination.

7. Was the brightness of the lights affected bychanging how the bulbs were hooked up? Explain.

Communicate8. Describe where you would find examples of a

series circuit and a parallel circuit.CAUTION: Open the circuit if the wires get hot.

• three or more flashlight bulbs with holders

• connecting wires

• three D dry cells in holders

• switches for each light


Figure 7.29 Circuits are carefully designed so that different electrical devices can be operated independently.

Key ActivityDI



C16 Skill Builder

Part 1 — Measuring CurrentMeasuring current involves measuring the amount ofelectric charge passing a given point in a given time.The current passes through the ammeter where it is measured. The ammeter is hooked in series intothe circuit, then the circuit is reconnected and themeasurement is taken. Follow these steps to hook up the ammeter.

1. Connect a battery and three resistors in seriesusing a resistor board. Open the circuit.

2. Hook your ammeter in series next to the positiveside of the battery. Be sure to connect the positive(red) terminal of the ammeter to the positive (�)terminal of the battery. Connect the negative (black)terminal of the ammeter to the negative (�)terminal of the battery (Figure 7.30). Record the reading.

3. Open the circuit and move the ammeter toimmediately beyond the first resistor. Repeat step 2.

4. Repeat step 3 for each resistor.

Using Equipment Accurately and Safely

6. To find the voltage across an electrical source,connect the meter by attaching the red lead tothe positive terminal and the black lead to thenegative terminal. This allows you to take a readingon both sides of the source. The meter indicatesthe difference in voltage between the two points.

7. To find the voltage across a resistor or load in acircuit, connect a lead to each side of the resistoror load. Connect the black lead closest to thenegative side of the source and the red leadclosest to the positive side of the source. Thismethod of connection is called connecting inparallel. By measuring voltage across the resistoror load, you are measuring the voltage drop asthe current moves through the resistor or load.

8. Use the voltmeter to test and report on the voltageof a variety of cells and batteries. (Note that 1.5-V batteries almost never actually read 1.5 V.)Compare your readings with the voltage numbersthat are written on their labels. If a multimeter isavailable, use it to repeat your measurementsand then compare the results.

9. Connect two or three dry cells in series. Do thisby placing them end to end with the positive endof one dry cell touching the negative end of theother dry cell. Predict the voltage reading, andthen use the voltmeter to check your prediction.

Figure 7.31 A voltmeter connected across a resistor

Figure 7.30 Connecting the ammeter in series

Part 2 — Measuring Voltage5. To insert a voltmeter in a circuit, connect the

two wires from the terminals of the voltmeter toopposite sides of the component for which youwant to measure the voltage (Figure 7.31).

CAUTION: Open the circuit if the wires get hot.




Initiating and PlanningWhat are the properties of a series circuit?

Series Circuit Analysis

SKILLS YOU WILL USE■ Observing and measuring■ Recording and organizing data

Toolkit 11

3. Record the voltage across each resistor and thepower supply.

4. Open the switch, and move the ammeter to aposition between the first two resistors. Close theswitch, and record the current leaving resistor 1.

5. Open the switch, and move the ammeter to aposition between the second and third resistors.Close the switch, and record the current leavingresistor 2.

6. Open the switch, and move the ammeter to aposition between the third resistor and thesource. Close the switch, and record the current leaving resistor 3.

Part 2 — Changing Resistance7. Open the switch, and remove one resistor. Close

the switch. Measure and record the current.

8. Measure and record the voltage across the powersupply and across each of the two resistors.

Analyzing and Interpreting9. State what you noticed in Part 1 about

(a) the current leaving the resistors in all cases

(b) the sum of all voltages across the resistors

10. State what happened in Part 2 to

(a) the current

(b) the voltages across each resistor

(c) the sum of the voltages across the resistors

11. What is the effect of adding an identical load inseries in a circuit?

12. Did the voltages across any resistors equal thetotal voltage provided by the source? Explain.

Communication and Teamwork13. Summarize the properties of a series circuit.

Performing and Recording

Part 1 — Measuring Voltage and Current1. Create a data table like the one below. Give your

table a title.

2. Construct the circuit shown in Figure 7.32. Keepthe switch open until your teacher approves yourcircuit. Then, close the switch and record thecurrent coming out of the power supply.

PowerSupply

Resistor1

Resistor2

Resistor3

Part 1:Current

Voltage

Part 2:Current

Voltage

Figure 7.32 Step 2 Construct this circuit.

6.0 V

resistor 1 resistor 2 resistor 3

V

A

• 6.0-V battery

• three 100-Ω resistors


• switch

• resistor board

• multimeter (or voltmeterand ammeter)


CAUTION: Open the circuit if the wires and resistors get hot.




Initiating and PlanningWhat are the properties of a parallel circuit?

Parallel Circuit Analysis

SKILLS YOU WILL USE■ Using appropriate equipment

and tools■ Recording and organizing data

3. Record the voltage across each resistor and thepower supply.

4. Open the switch, and move the ammeter to aposition between the first two resistors. Close theswitch, and record the current leaving resistor 1.

5. Open the switch, and move the ammeter to aposition between the second and third resistors.Close the switch, and record the current leavingresistor 2.

6. Open the switch, and move the ammeter to aposition between the third resistor and thesource. Close the switch, and record the current leaving resistor 3.

Part 2 — Changing Resistance7. Open the switch, and remove one resistor. Close

the switch. Measure and record the current.

8. Measure and record the voltage across the powersupply and across each of the two resistors.

Analyzing and Interpreting9. State what you noticed in Part 1 about

(a) the current leaving the resistors in all cases

(b) the sum of all voltages across the resistors

10. State what happened in Part 2 to

(a) the current

(b) the voltages across each resistor

(c) the sum of the voltages across the resistors

11. What is the effect of adding an identical load inparallel in a circuit?

12. Did the voltages across any resistors equal thetotal voltage provided by the source? Explain.

Communication and Teamwork13. Summarize the properties of a parallel circuit.

Performing and Recording

Part 1 — Measuring Voltage and Current1. Create a data table similar to the one below.

Give your table a title.

2. Construct the circuit shown in Figure 7.33. Keepthe switch open until your teacher approves yourcircuit. Then, close the switch and record thecurrent coming out of the power supply.

PowerSupply

Resistor1

Resistor2

Resistor3

Part 1:Current

Voltage

Part 2:Current

Voltage

Figure 7.33 Step 2 Construct this circuit.

6.0 V

resis

tor 1

resis

tor 2

resis

tor 3

V

A

• 6.0-V dry cell

• three 100-Ω resistors


• switch

• resistor board

• multimeter (or voltmeterand ammeter)


CAUTION: Open the circuit if the wires and resistors get hot.

Toolkit 11


A Fascination with ElectricityThe circuit boards in computers work because of the relationshipsbetween voltage, current, and resistance (Figure 7.34). Theserelationships have been understood for about 200 years becauseof the work of Georg Ohm.

Georg Simon Ohm (Figure 7.35) grew up in Germany in theearly 1800s. In high school, he studied physics, chemistry, math,and philosophy. He spent most of his free time playing billiards,ice skating, and dancing with his friends. No one imagined thatone day he would be a famous name in science.

After graduation, Ohm went to a private school in Switzerlandto teach. Here he taught mathematics and dreamed of studyingwith great mathematicians at an important university.

Ohm continued to study mathematics. One day, he was askedto instruct in the electricity labs. This was a turning point inGeorg Ohm’s life. Fascinated by electricity, he immersed himself inthe study of the characteristics of voltage, current, and resistance.



• Ohm’s law, V � IR, describesthe relationship betweenvoltage, current, and resistance.

• In a short circuit, the currentdoes not take the intendedpath back to its source.

• Fuses and circuit breakers aresafety devices.

Ohm’s Law

Figure 7.34 Voltage, current, and resistance have the same relationship in microcircuits in acomputer circuit board like this one as they do in the wiring in homes and offices.

7.3

Figure 7.35 Georg Ohm (1789–1854)

©P

Images SupportUnderstanding

As you read the text, be awareof how the photos, diagrams,or other illustrations supportyour understanding of newvocabulary. What term orconcept is illustrated by theimage? How does the imagemake the concept easier tounderstand? If you get stuckon unfamiliar terminology,check the images as one wayto improve your understanding.

During Reading


Use “I” for Current

The symbol “I ” is used forcurrent because it stands for “intensity.”

infoBITOhm’s passion and commitment to his studies led to a deepunderstanding of how these different electrical concepts wererelated. Much of what he discovered you have already learned in this unit. He stated these discoveries in what is today calledOhm’s law.

Ohm’s law established the relationships between voltage (V),current (I), and resistance (R). The symbol for resistance is calledthe ohm (Ω) in honour of Georg Ohm’s work in this field.

2597.3 Ohm’s Law©P

C19 Quick Science

Using the equipment available in your science class,you can investigate the same relationships betweenvoltage, current, and resistance that Georg Ohm did over 200 years ago.

PurposeTo measure how voltage, current, and resistance are related


2. Connect one resistor into a simple circuit. If youare using a voltmeter and ammeter, connectthese devices as well. Keep your circuit openuntil your teacher has approved your set-up.

3. Close your circuit.

4. Measure and record the voltage across the resistor.

5. Measure and record the current through theresistor.

6. Record the resistance of the resistor you used.

7. Repeat steps 2 to 6 for different resistors.

Questions8. Multiply the resistance by the current for each

of the trials you completed. What can you inferfrom your answers?

Pose New Questions9. If you were going to rewrite this activity, what

would you do differently to try and producedifferent results?

• 1.5-V dry cell

• resistors, any values from 15 Ω to 50 Ω


• switch

• resistor board

• multimeter or voltmeter and ammeter


Procedure1. Create a table like the following to record your

data. Give your table a title.

TrialResistance

(Ω)Current

(A)Voltage

(V)Resistance �

Current

1.

2.


Voltage, Current, and ResistanceGeorg Ohm described how voltage and current are affected whenone of the values is changed. He determined that the voltage (V) ina circuit is equal to the current (I) multiplied by the resistance (R).Ohm’s law states that, as long as temperature stays the same, V � IR (Figure 7.36). In other words,

• the resistance of a conductor remains constant

• the current is directly proportional to the voltage

Table 7.3 and the following examples show how to use Ohm’sLaw to calculate unknown quantities.


Suggested Activities •C20 Science, Technology, Society, andthe Environment on page 264C21 Inquiry Activity on page 265

V R

V = IR

I

V

Figure 7.36 Ohm’s law states thatvoltage (V) equals current (I) timesresistance (R).

KnownQuantity Symbol

UnknownQuantity Symbol Unit Equation

Current,resistance

IR voltage V V V � IR

Voltage,resistance

VR current I A I �VR

Voltage,current

VI resistance R Ω R �VI

Table 7.3 Ohm’s Law

Practice Problem

1. A current of 1.5 A flowsthrough a 30-Ω resistorthat is connected across a battery. Calculate thebattery’s voltage.

Example Problem 7.1

A current of 4.0 A flows through a 40-Ω resistor in a circuit.Calculate the voltage.

GivenCurrent I � 4.0 AResistance R � 40 Ω

RequiredVoltage V � x

Analysis and SolutionThe correct equation is V � IR.Substitute the values and their units, and solve the problem.

V � IR� (4.0 A)(40 Ω)� 160 V

ParaphraseThe voltage in the circuit is 160 V.



Example Problem 7.2

A 30-V battery generates a current through a 15-Ω resistor.How much current does the battery generate?

GivenVoltage V � 30 VResistance R � 15 Ω

RequiredCurrent I � x

Analysis and Solution

The correct equation is I � .

Substitute the values and their units, and then solve the problem.

I �VR

VR

Practice Problem

1. A firetruck has asearchlight with aresistance of 60 Ω that is placed across a 24-Vbattery. Calculate thecurrent in this circuit.

Example Problem 7.3

An electric stove is connected to a 240-V outlet. If the currentflowing through the stove is 20 A, what is the resistance of the heating element?

GivenVoltage V � 240 VCurrent I � 20 A

RequiredResistance R � x

Analysis and Solution

The correct equation is R � .

Substitute the values and their units, and then solve the problem.

R �VI

VI

Practice Problem

1. A current of 0.75 passesthrough a flashlight bulbthat is connected to a 3.0-V battery. Calculatethe bulb’s resistance.

� � 2 A

ParaphraseA current of 2 A is generated.

30 V15 Ω

� � 12 Ω

ParaphraseThe resistance of the heating element is 12 Ω.

240 V20 A


Short CircuitsSometimes, a wire’s insulation breaks down or another problemdevelops that allows electrons to flow through a device alonga different path than the one intended. The device develops ashort circuit. A short circuit is an accidental low-resistanceconnection between two points in a circuit, often causingexcess current flow (Figure 7.37). Not only do short circuitsmean that your electrical device will not work, they can alsobe dangerous. The conducting wires can quickly become hotfrom the excess current flow and can start a fire.

One danger from short circuits occurs when a transmissionline has been knocked down in a storm or accident. Without a complete path, the electrical energy cannot flow. However,if you come in contact with the wire, the electrical energy willtake a path through your body to the ground and seriouslyinjure or kill you. The driver shown in Figure 7.38 is safe as long as he stays inside the truck.

There are times when a technician must short out part of a circuit intentionally by connecting a wire across a load in parallel. The low-resistance wire causes the current toflow through it rather than through the higher resistancedevice. This allows the technician to work on the devicewithout interrupting the rest of the circuit.

Electrical SafetyAll electrical appliances present a risk ofelectric shock. Always handle electricalappliances properly and observe all safetyprecautions. Be careful to disconnect theplug before handling an appliance. Someelectronic devices, such as computers, retain electric charge even when they areunplugged (Figure 7.39). This is why many electrical devices have a “Do NotOpen” warning printed on them. Take the warning seriously, and do not attempt to repair the device yourself. Instead, contact a repair technician.


short circuit

Figure 7.39 Some electronic devices, such as this computer, store electrical energy even when the device is not plugged in.

Figure 7.38 The driver should stay in the truck and wait for help.

Figure 7.37 Current can flow more easilythrough the wire path than through the light bulb. This creates a short circuit, which could be dangerous.


Fuses and Circuit BreakersIn electric circuits in your home, fuses and circuitbreakers act as a first line of defence if something goeswrong. A fuse is a safety device in an electric circuitthat has a metallic conductor with a low melting pointcompared to the circuit’s wires (Figure 7.40). If thecurrent gets too high, the metal in the fuse melts, andthe current stops. This prevents further problems, suchas damage to your electrical components or a possiblefire. A blown fuse must be physically replaced as it can work only once. This symbol represents a fuse in a circuit diagram.

A circuit breaker does the same job as afuse except that the wire inside does not melt.Instead, the wire heats up and bends, whichtriggers a spring mechanism that turns off the flow of electrical energy. Once the breaker has cooled, it can be reset. Older homes and apartment buildings tend to have fusepanels, whereas modern buildings have breaker panels (Figure 7.41).

Three-Prong PlugAnother safety feature is the three-prongelectrical plug, shown in Figure 7.42. The third prong of a three-prong electrical plugconnects the device to the ground wire of thebuilding. The ground wire provides a directpath for any unwanted current to the ground.Instead of electrical energy travelling to themetal body of the device and shocking a person using it, the current is directed to the ground. The three-prong plug is nowstandard for most countries. In North Americaand much of South America, it is used forelectrical devices of 15 amperes at 125 volts.


Figure 7.40 Examples of fuses. A normal current canpass through a fuse, but a higher than normal currentor short circuit will melt the metal in the fuse, breakingthe circuit.

Figure 7.41 Circuit breakers help prevent electric overloads.

Figure 7.42 One prong in athree-prong plug carries thecurrent to the load, anotherprong returns the current to the source, and the third prongdirects the current to the groundin the case of a short circuit.


Ground Fault Circuit Interrupter Some appliances and devices have an added safety feature. A ground fault circuit interrupter (GFCI) or residual currentdevice is a device that detects a change in current and opens thecircuit, stopping the current (Figure 7.43). For example, if anappliance gets wet while you are handling it and some currentstarts to flow through the water, the GFCI opens the circuit sothere is less chance of injury to you. Remember, it is extremelydangerous to use any electrical device around water.


1. (a) How is current related to voltage in a circuit?

(b) How is current related to resistance in a circuit?

2. What does Ohm’s law state?

3. Copy this table into your notebook, andcalculate the missing values of the circuit.


5. What is the resistance in the circuit shown here?

Figure 7.43 Ground fault circuitinterrupters are part of some electric outlets.

C20

Electrical SafetyImagine you have just been hired by SaskPower to help create awareness of electrical safety forkindergarten students.

1. Research electrical safety concerns and, as a class, create a Home Electrical SafetyChecklist for home safety.

Science, Technology, Society, and the Environment

2. Create an electrical safety poster or brochurethat can be shared with a kindergarten class.Be sure to choose electrical safety points that arerelevant to young children and to communicatethese points in an engaging way.

6. What is each of these meters called andwhat does it measure?

3.0 A

6.0 V

Question 5

4. A 12-Ω bulb is in a series circuit poweredby a 6.0-V battery.(a) Calculate the current in the circuit.(b) If you changed the 12-Ω bulb to a

24-Ω bulb, what current would bedrawn from the battery?

(a) (b)

V I R

0.5 V 50 Ω

20 A 100 Ω

6.0 V 4.0 A


Question 6




Initiating and PlanningHow are voltage, current, and resistance related?

Investigating Ohm’s Law

SKILLS YOU WILL USE■ Analyzing patterns■ Drawing conclusions

Toolkit 11

3. Have your teacher approve your circuit, and thenclose the switch. Measure and record currentand voltage. Open the switch.

4. Replace resistor 1 with resistor 2. Repeat step 3.

5. Connect a second 1.5-V dry cell in series with the first cell in the circuit. Repeat steps 3 and 4,measuring current and voltage for each resistor.

6. Connect a third 1.5-V dry cell into the circuit.Repeat steps 3 and 4.

7. Connect a fourth 1.5-V dry cell. Repeat steps 3and 4.

8. Calculate your measured resistance for each

resistor using R � .

Analyzing and Interpreting9. (a) How did your calculated values for resistors

compare with their actual values?

(b) Explain possible reasons for any differencebetween the two values.

10. Compare your data for all resistor 1 trials. Whenvoltage is increased across a resistor, whathappens to the current?

11. Compare your data for all resistor 2 trials. Whenvoltage is increased across the resistor, whathappens to the current?

12. What would happen to the current values if youused a resistor with double the value of resistor 2?

Communication and Teamwork13. By graphing your results, describe the relationship

between voltage, current, and resistance. Shareyour graph with the rest of the class.

14. Explain how your group was able to work togethersafely to achieve your results.

VI

Performing and Recording1. Set up a data table like the one below. Fill in the

resistor value for the two resistors you will beusing. Examples below are 100 Ω and 200 Ω.Give your table a title.

CAUTION: Disconnect the circuit if the wires or resistorsget hot. Have your teacher check the circuit before youclose the switch or connect the power source.

Resistor(Ω)

Voltage(V)

Current(A)

CalculatedResistance

1.5 V1. 100

2. 200

3.0 V1. 100

2. 200

4.5 V1. 100

2. 200

6.0 V1. 100

2. 200

Figure 7.44 The secondcell, ammeter, andvoltmeter shown in thiscircuit diagram will beadded in step 5.V

A

2. Construct the following circuit using resistor 1and one 1.5-V dry cell (Figure 7.44).

• four 1.5-V dry cells


• voltmeter, ammeter

• switch

• two different resistorsbetween 100 Ω and300 Ω

• resistor board




Key Concept Review1. Is the circuit below a series circuit or a

parallel circuit? Explain why.

7 CHAPTER REVIEW

9. Explain the benefits and drawbacks ofprimary and secondary cells.

10. Assume that each resistor in a circuit is of a different value. What type of circuit doeseach of the following statements describe:series or parallel?

(a) The voltage is the same across everyresistor.

(b) The voltage varies across each resistor.

(c) The current varies through eachresistor.

(d) The current remains constantthroughout the whole circuit.

Connect Your Understanding11. Explain the reasons for each of these

safety rules.

(a) Do not poke a knife into a plugged-intoaster to clear out bread crumbs.

(b) Avoid using an extension cord that isthinner than the cord you are attachingto it.

(c) When disconnecting an appliance, pullthe plug, not the cord.

(d) Do not plug many electrical cords intoone outlet.

(e) Do not use a kite, stick, or pole close toan overhead wire.

(f) Make sure your hands are dry beforetouching any electrical device, cord,plug, or socket.

(g) Never use a frayed electrical cord.

©P

2. Draw a circuit diagram of a circuit thatincludes a battery, an ammeter, and a lightbulb with a voltmeter, all properlyconnected together.

3. How is a parallel circuit different from aseries circuit?

4. Are circuits in a home connected in series,in parallel, or in combinations? Explainyour answer, using examples of actualrooms in your home.

5. What is the difference between an opencircuit, a closed circuit, and a short circuit?

6. A current of 1.5 A flows through a 30-Ωresistor that is connected across a battery.Calculate the voltage of the battery.

7. A 120-V outlet has an appliance that draws10 A connected to it. What is the resistanceof the appliance?

8. Explain, with reference to electron transfer,how an electrochemical cell functions.

9.0 V

A1

4.0 V

V1

3.0 A

Question 1


12. (a) What is dangerous about the situationshown in the picture below?

(b) What should the worker do to be safer?

(c) The drill is plugged into the wall with athree-prong plug. How does the thirdprong on the plug act as a safetymechanism?

267Chapter 7 Review

Reflect and Evaluate

With a partner, list all the ways that this chaptersupports understanding of unfamiliar terms. Revisit your personal list of terms and definitions.Which terms are now more familiar to you? Whichterms might you need to review? What strategies will best help you to review those terms? Create two study goals for this chapter based on yourunderstanding of terms.

After Reading

Reflection on Essential Inquiry Questions

What do you understand about the relationshipsamong voltage, resistance, and currents?

How do parallel and series circuits affect howelectrical energy is distributed and used safely?

What are some costs and benefits of differenttypes of cells?

What knowledge of electricity do First Nationsand Métis peoples have?

Unit Task

In this chapter, you learned that current electricalenergy flows in a closed circuit. You also set upparallel and series circuits and learned aboutthe relationships that exist among voltage,current, and resistance. How can you applyyour new understanding to the Unit Task you have chosen?

CP

DM

TPS

SI

Reflection15. (a) What do you think is the most useful

information you learned in Chapter 7?Explain why.

(b) How might you put your understandingof this information to practical use?

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13. You want to find the value of an unlabelledresistor. You have a voltmeter, an ammeter,wires, and a battery. How could you findthe value of the resistor accurately?

14. An operational definition is one that explainsa process and includes clear instructions on how to measure and collect data. Forexample, the operational definition of theterm “weight” would provide instructions onhow an item would have its weight measured,including placing the item on a scale.Formulate an operational definition of

(a) voltage

(b) resistance

(c) current

Question 12


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